The present invention relates to Asynchronous Transfer Mode (ATM) transmissions. It specifically relates to providing such transmissions on synchronous wireless links.
ATM is a communication mode which enables numerous flows having various rate and quality of service (delay and error rates) characteristics to be multiplexed. For example, video flows are the most demanding streams since they have a high speed and require a relatively low error rate (in the order of 10xe2x88x926) with very strict delay requirements.
The basic data units transmitted in ATM are called cells and consist of packets made up of 53 octets, i.e. 48 octets of payload preceded by a 5-octet header.
Carrying a flow of ATM cells across a high-rate synchronous wireless interface gives rise to the following difficulties:
the wireless interface of the synchronous type must be capable of supporting an asynchronous data flow (ATM),
the size of the packets carried per timeslot on the radio interface does not generally correspond to that of the ATM cells,
the error rates which can be expected on such a wireless interface are much higher than what is desired for multimedia applications,
the delay incurred by the processing of the communication channel needs to be as short as possible in order to meet the strict requirements of certain applications in terms of transmission delay.
One of the types of wireless links to which the invention may be applied is that of DECT links (Digital Enhanced Cordless Telecommunications).
The DECT radio interface was originally devised as a wireless access to a fixed network supplying each user with a rate of 32 kbit/s without any protection, essentially intended for transmitting speech (the so-called non-protected mode). A protected mode also exists, which is intended for carrying data and which involves applying a block code for correcting errors, which has the disadvantage of requiring a complex decoding algorithm (decoding for BCH codes). By using a multi-slot allocation technique, the standard nevertheless permits symmetrical or asymmetrical high-rate wireless connections, each transmission direction of which is made up of a whole number of channels of 32 kbit/s without protection. Because of the simple channel coding, each wireless link offers an error rate of only approximately 10xe2x88x923.
As a result, using such links as a reliable means of carrying ATM cells for a variety of applications seems to be somewhat problematic.
A main object of the invention is to overcome this difficulty.
Accordingly, the invention proposes a interface device between an ATM equipment and a transmission channel having a synchronous wireless link. The interface device is arranged to process:
ATM cells;
segmented data units, each containing either data extracted from at least one ATM cell and alignment data locating the starts of ATM cells in said extracted data or stuffing data if no ATM cells are to be transmitted;
numbered data units, each containing a segmented data unit and numbering data, at least some of the numbered data units further containing acknowledgement data; and
protected data units having the size of packets which can be transmitted on the wireless link and each having a data field for containing a numbered data unit and an integrity verification field for containing an integrity verification code computed on the basis of said numbered data unit.
The device comprises, for the transmission of an ATM cell flow on the transmission channel:
a buffer memory receiving said ATM cell flow from the ATM equipment;
means for forming a first sequence of segmented data units, each containing data read from the buffer memory;
means for forming a second sequence of numbered data units, each containing a segmented data unit from the first sequence and numbering data generated in the sequence order of the segmented data units of the first sequence;
means for forming a third sequence of protected data units, each containing a numbered data unit from the second sequence and the integrity verification code computed for said numbered data unit, the sequence order of the protected data units in the third sequence being that of the numbered data units which they contain in the second sequence; and
means for transmitting the third sequence of protected data units on the transmission channel.
The interface device further comprises, for receiving an ATM cell flow from the transmission channel:
means for receiving a fourth sequence of protected data units from the transmission channel;
transmission error detection means for re-computing an integrity verification code on the basis of the contents of the data field of each protected data unit in the fourth sequence, comparing the re-computed code with the contents of the integrity verification field of said protected data unit and indicating a transmission error for said protected data unit if the comparison reveals a difference;
means of forming a fifth sequence of numbered data units respectively extracted from the protected data units of the fourth sequence for which no transmission error has been indicated;
means for forming a sixth sequence of segmented data units respectively extracted from the numbered data units of the fifth sequence; and
means for reconstructing an ATM cell flow output to the ATM equipment, on the basis of the data contained in the segmented data units of the sixth sequence, rearranged in accordance the numbering data contained in the numbered data units of the fifth sequence.
The means for forming the second sequence of numbered data units comprise means for inserting acknowledgement data in the numbered data units of the second sequence, whereby the acknowledgement data is indicative of the protected data units of the fourth sequence for which a transmission error has been indicated, and means for analysing the acknowledgement data contained in the numbered data units of the fifth sequence in order to insert in the second sequence of numbered data units repeats of numbered data units from the second sequence contained in protected data units of the third sequence for which the analysed acknowledgement data contained in the numbered data units of the fifth sequence indicate a transmission error.
Accordingly, the invention provides an adaptation layer between the ATM layer and the wireless link medium (MAC layerxe2x80x94Medium Access Control). It fulfils the following functions
1) Flow adaptation. The ATM flow transmission is asynchronous, whereas data transmission on the wireless interface is synchronous. Certain applications do not have specific rates (e.g. Internet) and can therefore transmit data at a rate higher than that permitted on the wireless medium. The interface device takes over the asynchronous/synchronous adaptation and rate control between the ATM flows and the flows transmitted on the wireless interface.
2) Data adaptation. The device transposes the ATM cells (53 octets) into packets that can be transmitted directly on the wireless interface (40 octets in the case of DECT). Similarly, at the receiving end, it reconstructs the original ATM cell flow from the packets received.
3) Data protection. In order to guarantee a certain transmission quality at the level of the wireless link, the device uses a data acknowledgement and retransmission mechanism (ARQ, Automatic Repeat reQuest) adapted to the characteristics of the synchronous wireless channel. A more common way of resolving the problem inherent in protecting data at the level of a wireless interface would be to protect the data to be transmitted using an error correction code. In a frequent situation where the channel is such that transmission errors occur by packets, the data has to be interleaved beforehand so that once the inverse operation has been performed at the receiving end (de-interleaving), the errors are dispersed and the decoder can operate correctly. There is a conflict between this operation, which is better the longer the interleaving depth, and the transmission delay requirement. Furthermore, error correction coding has the effect of reducing the useful rate since it adds redundancy to the data to be transmitted.
4) Providing Quality of Service (QoS). In order to match up to the quality of service requirements of the applications in terms of error rates and transmission delays, the device allows settings to be programmed for the data protection mechanism, for each ATM connection (service), depending on the characteristics of the service. The maximum permissible number of retransmissions and the type of acknowledgement of the ARQ mechanism can be used to offer the required QoS in terms of errors rates and delays.